Field of the Invention
The invention relates to the design, production and implementation of a lightweight biaxial flat plate concrete slab system, comprising semi-prefabricated elements, designed and produced in such a way, that post-tensioning of part of the system, facilitates a finished slab structure that is homogeneous and can be achieved without temporary supports during the execution.
Prior art lacks the ability to achieve homogeneous biaxial slabs without temporary supports, and the present invention solves these issues in a simple and economical manner. The enhanced range of applicability will lead to increased building speed, as well as environmental benefits through material reduction.
Description of the Prior Art
Concrete slabs can be regarded in three main groups based on the relevant criteria of function and execution: slabs fully concreted on site; fully precast elements or semi-precast elements. Each of these main groups can be divided into standard (soft steel) reinforced slabs or stressed hard steel slabs, solid or hollow/lightweight slabs, and one-way or two-way carrying slabs. The method of post-tension (PT) is used onsite at the finished concreted slab, while pre-tension is used in prefabrication. Of relevancy in relation to present system development is only the lightweight biaxial flat plate slab.
Slabs fully concreted on the building site demands scaffolding on which reinforcement can be placed and concreted. Such a slab cannot be pre-tensioned but post-tensioned by the use of tendons when concrete has hardened. After curing, the formwork can be removed. The essential weakness is the horizontal scaffolding and temporary vertical supports, which are expensive and time consuming.
Precast elements are full functional elements concreted 100% at factory and transported to the building site where to be erected without any temporary support. The weakness of fully pre-casted final elements are, that they per definition are one-way spanning elements and can only be used to achieve slabs spanning in one single direction, in contradiction to slabs concreted entirely or partly on the building site, which may be reinforced to carry in two directions. Fully precast elements are individual parts, and may have also problems with vibrations, sound and general leakage, why additional means normally are necessary.
Semi-precast elements are made on either factory or close to the building site, and normally comprises a bearing stiffening steel girder and a concrete bottom plate with basic reinforcement enabling the elements to carry their own weight in one direction during transport and implementation.
Semi-precast elements placed side by side can replace the horizontal part of the traditional formwork, and when concreted on site, after being finally reinforced, a homogeneous slab can be obtained—and as biaxial if continuously reinforced in both directions.
Even though the horizontal part of the formwork can be omitted by the use of semi-precast elements, the vertical part, the temporary supports, is still necessary, as the bearing capacity of the semi-precast elements normally is 1 to 2 m during concreting and hardening. The costs of temporary supports are 30-35% of the price for the final slab. Furthermore, the process is time consuming and demands labour for both erection and removing the supports.
In order to function, semi-precast elements have a concrete bottom of approximately 6 cm. This bottom can be applied with a weak pre-stressing, but the effect is limited, and this can only increase span between supports marginally, due to the limited height of the concrete bottom, which cannot be increased due to demands of minimising load and optimising space for voids. The essential problem is how to give a semi-precast element sufficient strength and stiffness to carry over large span—or same span as final slab—until final concreting has cured and working load can be added.
Steel profiles could be a theoretic solution, why attention is called to such examples.
Some patent applications [such as EP0794042] describe steel beams placed above the surface of a precast concrete panel and coupled to the concrete plate by various means. Placing the steel beam upon the plate opens for continuous steel reinforcing in the slab but the couplings cannot secure adequate transfer of the necessary forces between steel profile and concrete plate and besides, the effect of the steel beam itself will never be sufficient.
The lower flange of the steel beam is placed above the concrete plate and thus not encapsulated in the concrete plate. The remaining concrete cover is too thin to be stable and the contact surface between steel and concrete is too poor to transfer necessary shear forces. Increasing plate thickness is unthinkable and unrealistic as this will remove the basic idea of the slab type.
Facts are better than words—illustrated by an exact standard example with normal steel: Slab thickness 300 mm=> wanted slab span 30×thickness=9 m.
Available height=300-60−60=180 mm=> possible INP 180 (only slim profiles relevant) Disposable moment max M=W×f,yd=160000×180×10 E−6=29 kNm per profile. Slab load per 0.6 m (without safety factors) gives p=(7.2×0.75×0.6+0.2)=3.4 kN/m as no slabs have higher air-% than 25% (besides the BubbleDeck® technology as an exception). Actual design moment per profile is M=3.4×92/8=34 kNm and more than profile strength. Steel profiles closer than 0.6 m cannot longer perform a concrete slab, but is a one-way system of parallel steel beams that cannot in practice be integrated to compose a lightweight biaxial homogenous slab.
Patent application [PCT/KR2005/004320] confirms the mentioned weaknesses.
This application describes the use of steel beams with the lower flange encapsulated in massive (thick) heavy reinforced concrete to be able to transfer the necessary forces between concrete, applied pre-tensioned tendons, and soft steel profile to compose a unity and stronger beam.
However this application complies with only regular one-way beam structures without any possibility to compose a two-way continuous homogenous concrete slab and is therefore outside the field of the present invention.
All these applications incorporating steel beam profiles are highly impracticable and expensive in material consumption, as only a part of the steel has a function. However, the most important issue—if foot of steel profile is encapsulated in a thin concrete plate with 2 cm under and 2 cm above the steel foot—is that forces (in particular post-tension) cannot be secured transferred between the vulnerable thin concrete layers and the steel, because the concrete is not strong enough and if it was, it would require additional unpractical and expensive means like complicated anchors to secure the transfer.
Patent application [WO 97/14849] describes the possibility to make fully prefabricated element with steel beams, where the elements are being prepared to be connected onsite in the main direction by tensioning tendons drawn in ducts in lines above the columns. The structure thus composes a fully prefab regular one-way long spanning TT-beam.
The construction is not a biaxial homogeneous flat slab and is not semi-prefabricated to be casted on site, and is not within the field of the present invention.
The application describes “supporting steel beams” perpendicular to the main direction. These steel beams have no bearing effect, only to support the formwork below the lifted part of bottom surface, as the concrete flange easily can carry between the ribs when concreted and hardened. Further the formwork for bottom voids can be established much simpler and cheaper e.g. by polystyrene blocks.
Patent application [WO 00/53858] describes an onsite solution where multiple secondary beams are placed within short distance from each other on primary beams. Between the secondary beams are placed lightweight blocks, and when this system is concreted, a double ribbed slab is obtained with a main (beam) direction and a secondary (beam) direction and so with no relation to a homogeneous biaxial slab. Disadvantages are that it is a time consuming system made onsite; that the traditional beams can only span a relative short distance.
Patent application [EP1908891] describes a semi-precast slab element with ridges emerging in the main direction in its end areas. Due to this, the connected slab elements will compose a regular one-way structure without possibility for any two-way effect because continuity can only be established one way, due to the obstructing ridges at the sides. The construction does not substitute a biaxial homogeneous slab, and is outside the field of actual inventions.
Further, an essential problem with this invention is the use of ridge beams.
The fabrication and concreting of a semi-precast element incorporating such ridge beams results in problematic and expensive formwork as well as process, for which reason new system/methods are needed.
Furthermore, such a fabrication method excludes the possibility to have anything incorporated in the concrete extruding from the concrete in the same direction as the ridges compared to the panel, as the semi-precast elements with ridges necessarily must be made upside-down on the formwork. As a result, neither lattice girders, nor lightweight members etc. can be placed in the concrete prior to concreting. This results in an expensive element with limited function and no flexibility. Especially lightweight members as spheres must be placed in the openings in the reinforcement mesh placed in the semi-precast bottom, in order to combine optimal weight reduction with practical fixing, as defined by the BubbleDeck® technology. For this reason, new methods are needed.
In general, prior art only describe solutions with steel extruding upwards or downwards from the ridge relative to bottom plane of panel. An example is application [2325409]. No present production method enables steel extruding both upward and downwards as described in the present application.
Another used type of slab is standard semi-precast filigree elements, where the thin bottom is applied with pre-tensioned reinforcement. However, the effect is very limited due to the thin concrete bottom and do not comply with full dead load over realistic span.
Many applications describe the use of pre-tensioned beams. However, the use of pre-tensioning is ineffective, as the ability to transfer forces between beams and thin bottom plate is very limited. This consequently limits the tension with can be applied to the beams, and as a result limits the carrying effect—and leaves the carrying effect to the beams alone.
Further, the effective height is limited to the effective height within the pre-tensioned beam itself, which furthers reduces the effect.
Patent DE 202007007286 U describes such an idea using prefabricated pre-tensioned beams, which are to be placed partly in a thin concrete plate (not stressed) to form semi-prefabricated elements. Characteristics of this application are:                a. Pre-tensioned beams        b. Ability to transfer forces between beams and thin bottom plate is very limited        c. The carrying effect of the element is consequently identical to the carrying effect of the beam        d. Carrying effect of the beam is based on its internal height, from top of beam to main steel in beam—not to any steel in the plate, which limits the effect        e. Steel extending from the beam/concrete can not take part of the pre-tensioning, but will bend, and only function effectively as vertical connector during transport and handling        f. An advantage of traditional pre-tensioned prefab beams/elements is to introduce a curvature/cambering of the beam/element, but this ability is lost by this method due to the following concreting of plate        
To date, there exist no solutions with regards to voided homogeneous biaxial concrete flat slabs to be erected without the use of temporary supports. The building industry needs such solution.